This invention relates generally to cellular radiotelephone communications systems which may provide service to portable remote units and more particularly to the method and apparatus for determining whether the quality of the signal received from the portable remote unit is such that a change in the configuration of the radio path is necessary.
Mobile radiotelephone service has been provided for some time and traditionally has been characterized by a central site transmitting with high power to a limited number of mobile or portable remote units in a large geographic area. Mobile or portable transmissions, due to their lower transmission power, were generally received in previous systems by a network of receivers remotely located from the central site and the received transmission was subsequently returned to the central site for processing. In previous systems only a limited number of radio channels were available, thus limiting the number of radiotelephone conversations in an entire city to the limited number of channels available.
Modern cellular radiotelephone systems have a comparatively large number of radio channels available which, further, can be effectively multiplied by reuse of the channels in a metropolitan area by dividing the radio coverage area into smaller coverage areas (cells) using low power transmitters and coverage restricted receivers. Such cellular systems are further described in U.S. Pat. Nos. 3,906,166; 4,485,486; and 4,549,311, each assigned to the assignee of the present invention. The limited coverage area enables the channel frequencies used in one cell to be reused in another cell geographically separated according to a predetermined plan. Radio frequency energy may be transmitted from and received by a plurality of centrally located fixed stations and reuse of frequencies is accomplished in a predetermined repetitive pattern of cells. Alternative cell configurations and patterns may illuminate various sectors of a cell or may illuminate a cell from points other than from the center.
A cell system typically utilizes one duplex frequency pair channel in each cell (a signalling channel) to receive requests for service from mobile and portable remote units, to Call selected mobile or portable remote units, and to instruct the mobile or portable remote units to tune to another channel where a conversation may take place. This signalling channel is continuously assigned the task of receiving and transmitting data to control the actions of the remote units during call set-up.
Since the cells may be of relatively small size, the likelihood of a remote unit travelling between sectors or out of one cell and into another cell is high. The process of switching the established call from one sector or from one cell to another is known as handoff. Handoff generally requires specialized receiving equipment such as a "scanning" receiver which can be instructed to tune to any of the channels in use in any of the sectors of the cell to measure the signal strength of each active mobile or portable remote unit. If the measured signal strength or quality is below a predetermined level, cellular control equipment can take one of several steps to improve the quality of signal from the remote unit. The power being transmitted by the remote unit may be increased by a command transmitted by the control equipment on the channel in use. The antenna being used by the fixed equipment may be changed to a more optimum antenna (if the system design is so configured). Also, the remote unit may be handed off to another cell for continuation of the call on another frequency in the other cell.
A problem arises when a portable remote unit employs a transmitter which turns on and off depending upon whether a signal to be transmitted, e.g. voice, is present. A voice operated transmitter (VOX) significantly reduces the power drawn from a necessarily small portable remote unit battery and greatly prolongs battery life. However, if the control equipment requires a measurement of signal quality while the portable remote unit transmitter is off, an erroneous determination of signal quality will certainly be made.